The present invention relates to a method of controlling the forward and/or rearward movement of a percussion drill for producing holes in soil, including control phases of a reversible slide valve control arrangement. The present invention also relates to a percussion drill for carrying out such a method, with the drill comprising a control device for a flow medium for controlling movement of the drill in the forward and the rearward directions of travel, a housing having a percussion piston disposed in it, the percussion piston striking the housing and being adapted for reciprocating movement therein in response to pressure exerted by the flow medium and comprising an axial guide and lateraI control channels which cooperate with control channels of the control device wherein the control device is a rotary slide valve control device, comprising a control slide valve that is disposed for indexed or stepped rotation in a housing closure means and is connected to a flow medium feed pipe of the housing sealing device.
A first prior art percussion drill (DE-PS 26 34 066-Schmidt dated Sept. 20, 1984) comprises a partially hollow percussion piston that is adapted for reciprocating movement in a housing. A reversible control device for a flow medium extends into the hollow percussion piston and controls the movements thereof and thus indirectly also the forward or reverse movements of the percussion drill. The control device is seated in a screw-threaded end piece that seals off one end of the percussion drill housing and through which also the compressed air is supplied and discharged.
The control device is constructed so as to be integral with the screw-threaded end piece and has a stepped control tube, with that end thereof that extends into the hollow portion of the percussion piston, carrying a pot-shaped control sleeve in which are two diametrically disposed elongated axially parallel control slots. A first tubular part of the screw-threaded end piece engages over a thinner part of the control tube, while a second tubular part, constructed as an intermediate control sleeve, engages over the pot-shaped control sleeve, both tubular parts being connected via webs. The percussion piston slides in an axially reciprocating manner on this intermediate control sleeve. The stepped control tube is mounted for stepped rotation in the screw-threaded end piece and its intermediate control sleeve. For cooperation with the elongated control slits in the control tube, the intermediate control sleeve has matching slits, and immediately adjacent to them are discharge slits which make it possible, in appropriate switching positions, for exhaust air to flow out from the space between the percussion piston head and the housing through transverse ports at the end of the percussion piston. However, compressed air also flows through these transverse ports, through the elongated control slits upstream of the percussion piston head until the latter reverses its movement in the housing from the forward direction to the rearward direction. This compressed air brakes or arrests the percussion piston relatively abruptly at its front, dead center position upon reverse movement of the percussion drill, because the air pressure for moving the percussion piston in the direction of the percussion drill head drops abruptly and throughout the rest of the movement, and in return builds up similarly strongly and quickly upstream of the percussion piston head.
A second percussion drill (DE-PS 27 22 297-Tkac et al dated Mar. 22, 1984) which was developed after the first drill, comprises a partially hollow percussion piston that is adapted for reciprocating movement in a housing, and a reversible control device, for flow medium, which extends into the percussion piston and controls the movements thereof and thus indirectly also the forward or reverse movement of the percussion drill. The control device is seated in a screw-threaded end piece that seals off one end of the percussion drill housing, with the compressed air also being supplied and discharged through the end piece.
The control device has, seated in the screw-threaded end piece, a stepped and rotatable control tube comprising, on the end thereof which is received in the hollow portion of the percussion piston, an annular control step that projects on the outside surface and has on its free end face two diametrically disposed, elongated, axially parallel control slots which are open at the front end and on which, at the axially corresponding locations on the rear stepped end face, there are elongated control projections that have the same outside diameter as the annular control step. The percussion piston slides thereon in an axially reciprocating manner. Close to that end of the piston which is towards the screw-threaded end piece are two diametrically opposed transverse bores through which, in suitable switching positions, exhaust air can flow out of the space between the percussion piston head and the housing. However, compressed air also flows out through these transverse ports, through the elongated control slots, and upstream of the percussion piston head, until the piston head changes its direction of movement in the housing from the forward direction to the rearward direction. This compressed air which flows in permanently upstream of the percussion piston provides a relatively abrupt braking action for the piston.
This abrupt braking is a substantial disadvantage of both of the prior art percussion drills because in both cases considerable reaction forces act on the housing of the percussion drill during reverse movement, thus resulting in a very uneven and inefficient movement, particularly in soft ground. Furthermore, the compressed air consumption is very high since all of the compressed air which arrives in front of the percussion head flows out to the atmosphere through the screw-threaded end piece and is therefore lost. This constitutes a very high energy loss.
It is therefore an object of the present invention to improve the arrangements for controlling a percussion drill in such a way that the intense and abrupt loadings, and the compressed air consumption, are substantially reduced while at the same time the efficiency of the equipment is improved. It is also an object of the present invention to achieve a decided improvement in the efficiency of a percussion drill when driving a cable or bore hole, in other words during forward movement, by making more percussion energy available without thereby increasing the consumption of compressed air and without having to apply more energy to achieve a higher air pressure.